1. Field of the Invention
The present invention relates to an electric power generating system including a cell such as a fuel cell for outputting direct current power, and a conversion device having at least either an inverter for converting the direct-current power into alternating-current power or a converter for changing a voltage level of the direct-current power. More particularly, the invention relates to the art of improving energy efficiency of such electric power generating system.
2. Description of the Related Art
As such electric power generating system noted above, there is known a fuel cell electric generating system including a reformer for obtaining hydrogen from raw fuel material such as natural gas, a fuel cell for generating direct-current power through a reversed electrolysis of water between the hydrogen obtained by the reformer and oxygen obtained from air, with exhaust of high-temperature heat, and an exhaust heat recovery equipment for recovering thermal energy of the heat generated from the fuel cell.
Such fuel cell electric power generating system is an environmentally friendly electric power generating system due to its zero SOx emission, extremely low NOx emission, low CO.sub.2 emission, low generation of noises and vibrations. The system provides the further advantage of effective energy utilization of about 40% electric power generating efficiency, and about 85% total energy efficiency combining the electric power generating efficiency and the heat recovery efficiency. For these reasons, interest in the fuel cell electric power generating system as a new promising energy source has been ever increasing in recent years.
The inverter mentioned above generally comprises an electric circuit using diodes, transistors or the like which are formed of silicon semiconductor. Hence, the upper limit of the operating temperature of the inverter is determined by the upper limit of the operating temperature of the silicon semiconductor. The ambient temperature for rated operation of silicon semiconductor is generally 25.degree. C. And, the upper limit of the ambient temperature of the silicon semiconductor for general consumer use is 55.degree. C.
Incidentally, the fuel cell generates high-temperature exhaust heat in association with the chemical reaction for the electric power generation. Then, for effective utilization of this heat, the exhaust-heat recovery equipment is provided, as described above, for recovering the heat energy in the form of hot water or vapor. On the other hand, the operating temperature of the fuel cell is 60.degree. C. to 120.degree. C. if it is the solid high molecular type, 170.degree. C. to 220.degree. C. if it is the phosphoric acid type, 650.degree. C. if it is the molten carbon salt type, and even 1000.degree. C. if it is the solid electrolyte type, so that the fuel cell generates such high-temperature heat.
However, as the exhaust heat from the fuel cell normally has the temperature higher than 60.degree. C., the inverter cannot operate normally at such high temperature, hence leading to malfunction of the entire system. For enabling the inverter to operate at least at an ambient temperature below 60.degree. C., the prior art has proposed to shield the inverter from the high temperature or cool it and also to provide means for reducing the high-temperature thermal energy within the exhaust-heat recovery equipment. Such cooling is believed to be responsible for about 5% reduction in the total energy efficiency.
The present invention has been made in order to solve the above-described shortcoming of the convention. A primary object of the invention is to provide improvement in the energy efficiency of such electric power generating system.